The present invention relates to a photomask and a projection exposure apparatus, and more particularly, to a photomask formed with an original pattern to be transferred on a substrate in a lithography process for producing devices such as semiconductor integrated circuits, image pickup devices (CCDs etc.), liquid crystal displays and thin-film magnetic heads, etc. and to a projection exposure apparatus for transferring a predetermined pattern onto a substrate to be exposed using the photomask.
When devices such as the semiconductor integrated circuits are manufactured, a transfer method is used in which, using a photomask on which an original pattern obtained by scaling up a circuit pattern to be formed four to five times is formed, the pattern of this photomask is reduced and projected on the substrate to be exposed such as a wafer, a glass plate or the like through a reduction projecting optical system. An apparatus used when such a photomask pattern is transferred is the projection exposure apparatus. A photomask used in a step-and-repeat type reduction projection apparatus is also called as a reticle.
When such a photomask pattern is transferred, if foreign particles such as particles having a size exceeding tolerance are attached on a pattern surface, an image of the foreign particles is also transferred onto the substrate such as a wafer etc., and there is a fear that a finally produced integrated circuit and the like may malfunction. Thereupon, conventionally, an exposure light-transparent thin film called pellicle made of organic material having a thickness of about 1 to 10 xcexcm was provided, in a stretched manner, at a position away from the photomask pattern surface by 5 mm so that the pellicle prevented the foreign particles from attaching on the pattern surface. The pellicle was stretched over a metal supporting frame called a pellicle frame, a space between the pattern surface of the photomask and the pellicle was substantially cut off from outside air so that foreign particles included in the outside air would not attach to the pattern surface.
As described above, the pellicle made of a thin film of organic material was stretched over the conventional photomask pattern surface for dustproof. Further, in the conventional technique, i-line (wavelength of 365 nm) of a mercury lamp was mainly used as the exposure light of the projection exposure apparatus using the photomask. The conventional pellicle had sufficient endurance with respect to the exposure light of such a wavelength.
Recently, the wavelength of exposure light of the projection exposure apparatus tends to be shorter in order to cope with finer semiconductor integrated circuits, and a KrF excimer laser (wavelength of 248 nm) is becoming mainstream recently. Presently, an ArF excimer laser of shorter wavelength (wavelength of 193 nm) is becoming practical, and a research on the development of a projection exposure apparatus using F2 laser light of shorter wavelength (wavelength of 157 nm) is also conducted.
With such a shorter wavelength trend of exposure light, it has been difficult for the conventional organic pellicle to obtain sufficient endurance. That is, since exposure light of shorter wavelength has higher energy per one photon, chemical bonds of organic molecules constituting the pellicle are destroyed by the exposure light. For this reason, it is becoming difficult to provide a pellicle made of organic material having sufficient endurance against the exposure light of short wavelength.
However, if the pellicle is not used, foreign particles can not be prevented from attaching to the pattern surface of the photomask, and there is an inconvenience that yield of produced semiconductor integrated circuits etc. is lowered.
In view of the above circumstances, it is a first object of the present invention to provide a photomask having sufficient endurance against exposure beam of short wavelength and capable of preventing foreign particles from attaching to a pattern to be transferred.
Further, a second object of the present invention is to provide a projection exposure apparatus suitable for performing exposure using such a photomask.
Still further, a third object of the present invention is to provide an exposure method capable of preventing foreign particles from attaching to a pattern of a photomask and capable of transferring a finer pattern with high precision even if exposure beam of short wavelength is used, and to provide a method for producing a high-performance device using such an exposure method.
According to a first aspect of the present invention, there is provided a photomask, on which a transfer pattern is formed, for being irradiated with a predetermined exposure beam, comprising a pattern surface on which the pattern is formed, and a substrate being transparent with respect to the exposure beam, having a predetermined thickness, and being disposed such that the substrate is separated from the pattern surface at a predetermined distance via a supporting member.
According to the above-mentioned photomask, the substrate which is transparent with respect to the exposure beam, i.e., the substrate made of material including no organic material is used instead of the thin film (pellicle) made of organic material having a thickness of about 1 to 10 xcexcm which was conventionally used for preventing foreign particles from attaching. With this structure, it is possible to prevent the foreign particles from attaching, and to obtain the sufficient endurance against the exposure beam of short wavelength.
In the present invention, when the exposure beam is of ultraviolet light having a wavelength of about 100 nm to about 300 nm, it is preferable to seal a space surrounded by the substrate, the supporting member and the pattern surface, and to fill the sealed space with a nitrogen gas, a helium gas or a hydrogen gas.
In this case, since, for example, ArF excimer laser light is greatly absorbed by oxygen, if the sealed space is filled with a gas that does not absorb ultraviolet rays so much such as a nitrogen gas, a helium gas and a hydrogen gas, an attenuation of exposure beam is decreased, and the exposure beam can be utilized more efficiently. Further, helium gas is safe and hydrogen gas is inexpensive.
Further, the distance between the substrate and the pattern surface is preferably about 2 mm to about 10 mm, and the thickness of the substrate is preferably about 0.05 mm to about 2 mm. If the distance is narrower than 2 mm, an image of foreign particles attached to a surface of the dustproof substrate may adversely be transferred on the substrate to be exposed relatively clearly. On the other hand, if the distance exceeds 10 mm, when the photomask is placed in the projection exposure apparatus, the transparent substrate may adversely interfere with a stage and the like mechanically, and this is not preferable. If the thickness of the transparent substrate is thinner than 0.05 mm, the substrate may be damaged by a thermal deformation of a substrate of the photomask itself, and if the thickness exceeds 2 mm, a correction amount of image-forming characteristics of a projector optical system using the photomask may be excessively increased, and this is not preferable.
When a wavelength of the exposure beam is about 100nm to about 180 nm, the substrate is preferably made of any one of silica glass (SiO2), fluorite (CaF2), magnesium fluoride (MgF2), quartz, lithium fluoride (LiF), lanthanum fluoride (LaF3), and lithium-calcium-aluminum-fluoride (commonly called xe2x80x9cLiCAF crystalxe2x80x9d).
Organic material is easily damaged by ultraviolet light having wavelength of about 100 nm to about 300 nm, but the above-mentioned silica glass and fluorite, etc. can be used also as lens material for a projection optical system using the ArF excimer laser light (wavelength of 193 nm), and have sufficient endurance against exposure beam radiation of such short wavelength. As the silica glass, it is possible to use not only a normal synthetic quartz but also a synthetic quartz having fluorine.
It is preferable that the photomask includes a protecting member (for example, such as a metal film) for covering adhesive or filler for sealing between the substrate and the supporting member or between the pattern surface and the supporting member. When the photomask includes the protecting member, it is possible to prevent impurities (for example, such as silicon-based organic substance) from being generated from the adhesive or filler, and to prevent transmissivity of the substrate and the photomask substrate from being deteriorated, which is caused when the impurities attach to the substrate and the photomask substrate or enter (float into) an illumination light path.
Next, according to a second aspect of the present invention, there is provided an exposure method, comprising a step of transferring, with a predetermined exposure beam, a pattern of a mask onto a substrate to be exposed, wherein the mask includes a pattern surface and a substrate, the pattern being formed on the pattern surface, the substrate being transparent with respect to the exposure beam and being disposed such that the substrate is separated, at a predetermined distance, from the pattern surface, and a space between the substrate and the pattern surface being substantially sealed.
According to the above exposure method, even if an exposure beam of short wavelength is used, it is possible to prevent foreign particles from attaching to the pattern surface of the mask and to transfer a finer pattern with high precision.
When the exposure beam is of ultraviolet light having a wavelength of about 100 to about 300 nm, it is preferable to fill the sealed space with a nitrogen gas, a helium gas or a hydrogen gas. The distance between the substrate and the pattern surface is preferably about 2 mm to about 10 mm, and the thickness of the substrate is preferably about 0.05 mm to about 2 mm.
According to a third aspect of the present invention, there is provided a method for manufacturing a device, comprising the exposure step of transferring, using the above-mentioned exposure method according to the present invention, an image of the pattern of the mask onto the substrate to be exposed.
With the method, it is possible to transfer a finer pattern with high precision and to produce a high-performance device.
A thickness of the transparent substrate of the photomask of according to the present invention is extremely thick as compared with the thickness (1 to 10 xcexcm) of the conventionally used pellicle made of an organic thin film, and an aberration, especially a spherical aberration generated when an image-forming luminous flux passes through the substrate becomes great enough not to be ignored.
Thereupon, in the projection exposure apparatus according to the present invention using the photomask having the dustproof transparent substrate, it is preferable to design and produce the projection optical system taking the aberration generated by the transparent substrate into consideration from the design phase of the projection optical system.
That is, according to a fourth aspect of the present invention, there is provided a projection exposure apparatus for handling the above-mentioned photomask, comprising an illumination optical system for illuminating the photomask with the exposure beam, and a projection optical system for projecting an image of the pattern of the photomask onto the substrate to be exposed with the exposure beam passing through the pattern surface of the photomask and the transparent substrate, wherein an image-forming state of the projection optical system is adjusted in accordance with at least one of the thickness of the transparent substrate of the photomask and the distance between the substrate and the pattern surface.
With this feature, even the image of the pattern on the photomask having such a thick transparent substrate can precisely be transferred to the substrate to be exposed.
Further, the above-mentioned projection exposure apparatus according to the present invention is produced by mechanically and optically assembling the above-described various members so that an aberration caused by the substrate can be compensated.
In this case, it is preferable to provide an image-forming state adjusting mechanism for adjusting the image-forming state of the projection optical system in accordance with at least one of the thickness of the transparent substrate of the photomask and the distance between the substrate and the pattern surface. At that time, even if the thickness of the transparent substrate is varied among the transparent substrates due to such as errors caused during production, by adjusting the image-forming state of the projection optical system in accordance with the thickness of each of the dustproof transparent substrates of the photomasks to be exposed, it is possible to precisely transfer each of the patterns of corresponding photomasks.
It is preferable to design and produce the projection optical system in accordance with at least one of the thickness of the substrate and the distance between the substrate and the pattern surface, and an aberration of the projection optical system caused by the substrate is corrected.
According to a fifth aspect of the present invention, there is provided an exposure apparatus for transferring, with an exposure beam, an image of a pattern formed- on a mask onto a substrate to be exposed, comprising an adjusting mechanism, wherein the mask includes a substrate for protecting a pattern surface on which the pattern is formed, the substrate being transparent with respect to the exposure beam and having a predetermined thickness, and the adjusting mechanism adjusts an image-forming state of the image of the pattern with consideration given to the predetermined thickness of the substrate.
According to a sixth aspect of the present invention, there is provided an exposure method, comprising a step of transferring, with an exposure beam, an image of a pattern formed on a mask onto a photosensitive substrate, wherein the mask includes a substrate, the substrate being disposed such that the substrate is separated, at a predetermined distance via a supporting member, from a: pattern surface on which the pattern is formed, being transparent with respect to the exposure beam and having a predetermined thickness, and an image-forming state of the image of the pattern is adjusted with consideration given to at least one of the predetermined thickness of the substrate and the distance between the substrate and the pattern surface.